Transient increase in the transcript levels of gamma-tubulin complex genes during reorientation of cortical microtubules by gravity in azuki bean (Vigna angularis) epicotyls

Involvement of KATANIN1, a microtubule-severing enzyme, in hypergravity-induced modification of growth anisotropy in Arabidopsis hypocotyls

The mechanism by which gravity affects the growth anisotropy of plant organs is an important issue for the cultivation of plants under microgravity conditions in space. This study aimed to examine the roles of KATANIN1 (KTN1), a microtubule-severing enzyme, in the modification of the direction of cortical microtubules (CMTs) and growth anisotropy in Arabidopsis hypocotyls induced by gravity using hypergravity conditions that can be created on Earth. The KTN1 mutants (ktn1P393S, ktn1R465Q, and ktn1S342F) exhibited shorter and thicker hypocotyls than the wild type (ecotype Columbia-0). Hypergravity at 300 g modified growth anisotropy in wild-type hypocotyls; hypergravity inhibited elongation but stimulated lateral growth. In contrast, hypergravity-induced modification of growth anisotropy was suppressed in hypocotyls of the three mutants. The wild type had an abundance of CMTs in transverse orientation (between 0° and 30°) under 1 g conditions and a tendency toward increased CMTs in longitudinal orientation under hypergravity conditions (between 60° and 90°). However, hypergravity-induced reorientation was not observed in hypocotyls of ktn1 mutants. The transcript level of the KTN1 gene in wild-type hypocotyls increased within 1 h of onset of hypergravity treatment and promptly decreased to the same level as the 1 g control. These findings suggest that the reorientation of CMTs is mediated by KTN1, which is regulated by transient expression upregulation, which is responsible for the modification of growth anisotropy induced by hypergravity in Arabidopsis hypocotyls.

Plant responses to hypergravity: a comprehensive review

Hypergravity is an effective novel stimulus to elucidate plant gravitational and mechanobiological behaviour. Here, we review the current understanding of phenotypic, physio-biochemical, and molecular plant responses to simulated hypergravity. Plants readily respond to altered gravity conditions, such as microgravity or hypergravity. Hypergravity-a gravitational force higher than that on the Earth's surface (> 1g)-can be simulated using centrifuges. Exposing seeds, seedlings, or plant cell cultures to hypergravity elicits characteristic morphological, physio-biochemical, and molecular changes. While several studies have provided insights into plant responses and underlying mechanisms, much is still elusive, including the interplay of hypergravity with gravitropism. Moreover, hypergravity is of great significance for mechano- and space/gravitational biologists to elucidate fundamental plant behaviour. In this review, we provide an overview of the phenotypic, physiological, biochemical, and molecular responses of plants to hypergravity. We then discuss the involvement of hypergravity in plant gravitropism-the directional growth along the gravity vector. Finally, we highlight future research directions to expand our understanding of hypergravity in plant biology.

An Arabidopsis PTH2 Gene Is Responsible for Gravity Resistance Supporting Plant Growth under Different Gravity Conditions

Terrestrial plants respond to and resist gravitational force. The response is termed "gravity resistance", and centrifugal hypergravity conditions are efficient for investigating its nature and mechanism. A functional screening of Arabidopsis T-DNA insertion lines for the suppression rate of elongation growth of hypocotyls under hypergravity conditions was performed in this study to identify the genes required for gravity resistance. As a result, we identified PEPTIDYL-tRNA HYDROLASE II (PTH2). In the wild type, elongation growth was suppressed by hypergravity, but this did not happen in the pth2 mutant. Lateral growth, dynamics of cortical microtubules, mechanical properties of cell walls, or cell wall thickness were also not affected by hypergravity in the pth2 mutant. In other words, the pth2 mutant did not show any significant hypergravity responses. However, the gravitropic curvature of hypocotyls of the pth2 mutant was almost equal to that of the wild type, indicating that the PTH2 gene is not required for gravitropism. It is suggested by these results that PTH2 is responsible for the critical processes of gravity resistance in Arabidopsis hypocotyls.

Suppression of Cortical Microtubule Reorientation and Stimulation of Cell Elongation in Arabidopsis Hypocotyls under Microgravity Conditions in Space

How microgravity in space influences plant cell growth is an important issue for plant cell biology as well as space biology. We investigated the role of cortical microtubules in the stimulation of elongation growth in Arabidopsis (Arabidopsis thaliana) hypocotyls under microgravity conditions with the Resist Tubule space experiment. The epidermal cells in the lower half of the hypocotyls of wild-type Columbia were longer in microgravity than at on-orbit 1 g, which precipitated an increase in the entire hypocotyl length. In the apical region, cortical microtubules adjacent to the outer tangential wall were predominantly transverse to the long axis of the cell, whereas longitudinal microtubules were predominant in the basal region. In the 9th to 12th epidermal cells (1 to 3 mm) from the tip, where the modification of microtubule orientation from transverse to longitudinal directions (reorientation) occurred, cells with transverse microtubules increased, whereas those with longitudinal microtubules decreased in microgravity, and the average angle with respect to the transverse cell axis decreased, indicating that the reorientation was suppressed in microgravity. The expression of tubulin genes was suppressed in microgravity. These results suggest that under microgravity conditions, the expression of genes related to microtubule formation was downregulated, which may cause the suppression of microtubule reorientation from transverse to longitudinal directions, thereby stimulating cell elongation in Arabidopsis hypocotyls.

MCA1 and MCA2 Are Involved in the Response to Hypergravity in Arabidopsis Hypocotyls

Plants respond to and resist gravitational acceleration, but the mechanism of signal perception in the response is unknown. We studied the role of MCA (mid1-complementing activity) proteins in gravity perception by analyzing the expression of the MCA1 and MCA2 genes, and the growth of hypocotyls of mca mutants, under hypergravity conditions in the dark. An MCA1 promoter::GUS fusion reporter gene construct (MCA1p::GUS) and MCA2p::GUS were expressed almost universally in etiolated seedlings. Under hypergravity conditions, the expression levels of both genes increased compared with that under the 1 g condition, and remained higher, especially in the basal supporting region. On the other hand, mca-null and MCA-overexpressing seedlings showed normal growth under the 1 g condition. Hypergravity suppressed elongation growth of hypocotyls, but this effect was reduced in hypocotyls of mca-null mutants compared with the wild type. In contrast, MCA-overexpressing seedlings were hypersensitive to increased gravity; suppression of elongation growth was detected at a lower gravity level than that in the wild type. These results suggest that MCAs are involved in the perception of gravity signals in plants, and may be responsible for resistance to hypergravity.

Growth and cortical microtubule dynamics in shoot organs under microgravity and hypergravity conditions

The body shape of plants varied in proportion to the logarithm of the magnitude of gravity in the range from microgravity to hypergravity to resist the gravitational force. Here we discuss the roles of cortical microtubule and 65 kDa microtubule-associated protein-1 (MAP65-1) in gravity-induced modification of growth anisotropy. Microgravity stimulated elongation growth and suppressed lateral expansion in shoot organs, such as hypocotyls and epicotyls. On the other hand, hypergravity inhibited elongation growth and promoted lateral expansion in shoot organs. The number of cells with transverse microtubules was increased by microgravity, but decreased by hypergravity. Furthermore, the levels of MAP65-1, which is involved in the maintenance of the transverse microtubule orientation, were increased by microgravity, but decreased by hypergravity. Therefore, the regulation of orientation of cortical microtubules via changes in the levels of MAP65-1 may contribute to the modification of the body shape of plants to resist the gravitational force.

Gene co-expression analyses: an overview from microarray collections in Arabidopsis thaliana

Bioinformatics web-based resources and databases are precious references for most biological laboratories worldwide. However, the quality and reliability of the information they provide depends on them being used in an appropriate way that takes into account their specific features. Huge collections of gene expression data are currently publicly available, ready to support the understanding of gene and genome functionalities. In this context, tools and resources for gene co-expression analyses have flourished to exploit the 'guilty by association' principle, which assumes that genes with correlated expression profiles are functionally related. In the case of Arabidopsis thaliana, the reference species in plant biology, the resources available mainly consist of microarray results. After a general overview of such resources, we tested and compared the results they offer for gene co-expression analysis. We also discuss the effect on the results when using different data sets, as well as different data normalization approaches and parameter settings, which often consider different metrics for establishing co-expression. A dedicated example analysis of different gene pools, implemented by including/excluding mutant samples in a reference data set, showed significant variation of gene co-expression occurrence, magnitude and direction. We conclude that, as the heterogeneity of the resources and methods may produce different results for the same query genes, the exploration of more than one of the available resources is strongly recommended. The aim of this article is to show how best to integrate data sources and/or merge outputs to achieve robust analyses and reliable interpretations, thereby making use of diverse data resources an opportunity for added value.

Inclination not force is sensed by plants during shoot gravitropism

Gravity perception plays a key role in how plants develop and adapt to environmental changes. However, more than a century after the pioneering work of Darwin, little is known on the sensing mechanism. Using a centrifugal device combined with growth kinematics imaging, we show that shoot gravitropic responses to steady levels of gravity in four representative angiosperm species is independent of gravity intensity. All gravitropic responses tested are dependent only on the angle of inclination from the direction of gravity. We thus demonstrate that shoot gravitropism is stimulated by sensing inclination not gravitational force or acceleration as previously believed. This contrasts with the otolith system in the internal ear of vertebrates and explains the robustness of the control of growth direction by plants despite perturbations like wind shaking. Our results will help retarget the search for the molecular mechanism linking shifting statoliths to signal transduction.

Role of the plant cell wall in gravity resistance

Gravity resistance, mechanical resistance to the gravitational force, is a principal graviresponse in plants, comparable to gravitropism. The cell wall is responsible for the final step of gravity resistance. The gravity signal increases the rigidity of the cell wall via the accumulation of its constituents, polymerization of certain matrix polysaccharides due to the suppression of breakdown, stimulation of cross-link formation, and modifications to the wall environment, in a wide range of situations from microgravity in space to hypergravity. Plants thus develop a tough body to resist the gravitational force via an increase in cell wall rigidity and the modification of growth anisotropy. The development of gravity resistance mechanisms has played an important role in the acquisition of responses to various mechanical stresses and the evolution of land plants.

Growth stimulation in inflorescences of an Arabidopsis tubulin mutant under microgravity conditions in space

Cortical microtubules are involved in plant resistance to hypergravity, but their roles in resistance to 1 g gravity are still uncertain. To clarify this point, we cultivated an Arabidopsis α-tubulin 6 mutant (tua6) in the Cell Biology Experiment Facility on the Kibo Module of the International Space Station, and analyzed growth and cell wall mechanical properties of inflorescences. Growth of inflorescence stems was stimulated under microgravity conditions, as compared with ground and on-orbit 1 g conditions. The stems were 10-45% longer and their growth rate 15-55% higher under microgravity conditions than those under both 1 g conditions. The degree of growth stimulation tended to be higher in the tua6 mutant than the wild-type Columbia. Under microgravity conditions, the cell wall extensibility in elongating regions of inflorescences was significantly higher than the controls, suggesting that growth stimulation was caused by cell wall modifications. No clear differences were detected in any growth or cell wall property between ground and on-orbit 1 g controls. These results support the hypothesis that cortical microtubules generally play an important role in plant resistance to the gravitational force.

Resistance of plants to gravitational force

Developing resistance to gravitational force is a critical response for terrestrial plants to survive under 1 × g conditions. We have termed this reaction "gravity resistance" and have analyzed its nature and mechanisms using hypergravity conditions produced by centrifugation and microgravity conditions in space. Our results indicate that plants develop a short and thick body and increase cell wall rigidity to resist gravitational force. The modification of body shape is brought about by the rapid reorientation of cortical microtubules that is caused by the action of microtubule-associated proteins in response to the magnitude of the gravitational force. The modification of cell wall rigidity is regulated by changes in cell wall metabolism that are caused by alterations in the levels of cell wall enzymes and in the pH of apoplastic fluid (cell wall fluid). Mechanoreceptors on the plasma membrane may be involved in the perception of the gravitational force. In this review, we discuss methods for altering gravitational conditions and describe the nature and mechanisms of gravity resistance in plants.

Transient increase in the levels of γ-tubulin complex and katanin are responsible for reorientation by ethylene and hypergravity of cortical microtubules

The body shape of a plant is primarily regulated by orientation of cortical microtubules. γ-Tubulin complex and katanin are required for the nucleation and the severing of microtubules, respectively. Here we discuss the role of γ-tubulin complex and katanin during reorientation of cortical microtubules. 1-Aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, modifies growth anisotropy of azuki bean epicotyls; it inhibits elongation growth and promotes lateral growth. The ACC-induced reorientation of cortical microtubules from transverse to longitudinal directions preceded the modification of growth anisotropy. The transcript level of γ-tubulin complex (VaTUG and VaGCP3) and katanin (VaKTN1) was increased transiently by ACC treatment. During reorientation of cortical microtubules by hypergravity, which also modifies growth anisotropy of shoots, the expression levels of both γ-tubulin complex and katanin genes were increased transiently. The increase in the number of the nucleated microtubule branch as well as the microtubule-severing activity via upregulation of γ-tubulin complex genes and katanin genes may be involved in the reorientation of cortical microtubules, and contribute to the regulation of the shape of plant body.

1-aminocyclopropane-1-carboxylic acid (ACC)-induced reorientation of cortical microtubules is accompanied by a transient increase in the transcript levels of gamma-tubulin complex and katanin genes in azuki bean epicotyls

The effects of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, on growth, orientation of cortical microtubules, and the transcript levels of gamma-tubulin complex (VaTUG and VaGCP3) and katanin (VaKTN1) genes in azuki bean (Vigna angularis) epicotyls were examined. ACC inhibited elongation growth and stimulated lateral growth of epicotyls dose dependently. It also reduced the percentage of cells with transverse microtubules and increased the percentage of cells with longitudinal microtubules. A significant change in elongation and lateral growth was detected within 1 and 1.5 h after the start of 10(-5) M ACC treatment, respectively. On the other hand, the reorientation of cortical microtubules from transverse to longitudinal direction began within 0.5 h, and continued until 2 h after the start of ACC treatment. ACC at 10(-5) M increased the transcript level of VaTUG, VaGCP3 and VaKTN1 within 0.5 h, and the levels of VaTUG and VaGCP3 became maximum at 1h and that of VaKTN1 at 1.5 h, followed by a decrease to the control level. These results suggest that ACC transiently increases the transcript levels of gamma-tubulin complex and katanin genes, which may facilitate reorientation of cortical microtubules and modification of growth anisotropy from elongation to lateral growth in azuki bean epicotyls.

Cortical microtubules are responsible for gravity resistance in plants

Mechanical resistance to the gravitational force is a principal gravity response in plants distinct from gravitropism. In the final step of gravity resistance, plants increase the rigidity of their cell walls. Here we discuss the role of cortical microtubules, which sustain the function of the cell wall, in gravity resistance. Hypocotyls of Arabidopsis tubulin mutants were shorter and thicker than the wild-type, and showed either left-handed or right-handed helical growth at 1 g. The degree of twisting phenotype was intensified under hypergravity conditions. Hypergravity also induces reorientation of cortical microtubules from transverse to longitudinal directions in epidermal cells. In tubulin mutants, the percentage of cells with longitudinal microtubules was high even at 1 g, and it was further increased by hypergravity. The left-handed helical growth mutants had right-handed microtubule arrays, whereas the right-handed mutant had left-handed arrays. Moreover, blockers of mechanoreceptors suppressed both the twisting phenotype and reorientation of microtubules in tubulin mutants. These results support the hypothesis that cortical microtubules play an essential role in maintenance of normal growth phenotype against the gravitational force, and suggest that mechanoreceptors are involved in signal perception in gravity resistance. Space experiments will confirm whether this view is applicable to plant resistance to 1 g gravity, as to the resistance to hypergravity.